In recent years there has been a proliferation of communication facilities involving many remote stations and terminals working together with data processors in a network. Generally, such network systems involve a host processor working with a main memory to form a central processing unit, or even a plurality of such central processing units, whereby digitized message data can be transmitted from one station or terminal to another station or terminal within the system, but which, of course, the transmission must be routed, controlled and organized to accomplish the message transfer in an orderly and accurate fashion.
In the field of data communications each data transmission line is connected to a "line adapter" which interfaces the data communications line into the system network. These line adapters may be associated together in a group and called an Adapter Cluster or, that is to say a group or cluster of adapters physically located within one unit. Each line adapter is specifically designed to operate to suit the characteristics of a particular type of remote terminal or station. The line adapter has to take into account factors such as the type of characters transmitted, the coding type of characters, the type of parity that is used, whether transmission is synchronous or asynchronous, the data rate or speed of transmission permissable, and so on, in order to provide that the terminal station connected at the other end of the transmission line will receive the proper type of signals.
Efforts are continuously being made to increase throughput, i.e., the number of message bits that can accurately be transmitted per unit time while minimizing the cost of equipment and facilities for accomplishing this. However, there must also be flexibility, in that provision must be made for wide band high speed transmission lines for high speed transmission of data, in addition to low to medium speed transmission lines which are commonly used since they are cheaper in cost. Further, the accessibility of message data stored in memory must be speedily available in order to obviate delays and increase throughput, and the desirability of concurrent overhead control operations to reduce delays has been recognized.
The field of this invention pertains to data processing equipment which is intended for use with a wide variety of remotely located terminal devices. It has become very desirable to incorporate a data processing system into a network for transmission of data over long distances. The terminal devices involved will generally convert the data from a humanly readable form into binary digital form and transmit this data over wires or microwave relay systems. The terminal devices operate under and generate a wide variety of message code sets, character lengths, bit rates, message formats, communication line disciplines and modes of transmission which present considerable problems to the designer of data communication equipment. The data communication equipment must be able to interface with a wide variety of different types of these terminal devices and should be flexible enough that additional devices can be added or that the terminal devices already used can be changed according to customer preference.
Many of the past and presently existing data communication systems are categorized by those systems which are designed with fixed hardware and are intended to interface only with a specific type of terminal device. This may be economical but is not particularly flexible; other systems have been designed in a modular form to provide options for each of the modules to provide compatibility with certain types of terminal equipment. Because of the differences required among different line disciplines and different types of terminal requirements, it is not usually possible to design a common logic system to perform control functions to cover each of the variety of types of terminals. Among the difficulties involved is that of providing a comprehensive software package to service different configurations and in which the configurations may be desired to be changed from time to time. Thus, in the economics of time and hardware it has often been found necessary to limit the software to one particular type of data communication lines and terminal stations in the system.
With the development of integrated circuits and mini and microcomputers, it is now possible to provide hardware and software of great flexibility in order to handle systems which may have many possible configurations and newly desired configurations in the future. Often it was necessary that a particular program or subroutine be provided for each type of terminal device connected to the system and when new terminals were added to the system, a new subroutine was provided. This activity, however, lead to considerable expense, in addition to eating up long periods of time within the processor.
The present invention overcomes many of the earlier limitations and provides faster throughput of data transfers while permitting reconfigurability and also adaptability to various type of transmission lines and terminal equipment characteristics.
The presently described data communication subsystem has the objective of optimizing the message transference and handling between sending and receiving terminals in a data communication system network and to optimize the data communication transfer as between a computer or computers and the terminals; to provide direct memory access at the message level by providing a larger data communication memory; to provide self-organizing configurations together with a continuous operation system; to provide a temporary storage facility such as disks which can permit the "tanking" of messages in order to provide backup storage for the system; and to provide high-speed, computer-to-computer interface capability.